The behavioral consequences of experimentally-induced alterations of thyroid function are being examined in monkeys and rats. Primary emphasis is on performance in learning situations by adolescent or adult animals which have been subjected to severe hypothyroid or hyperthyroid states during critical periods of early development, but attention is also directed toward the maturation of motor, emotional, and social capacities during infancy in these animals and the effects of altered thyroid states during adulthood upon emotional and cognitive functioning. Manipulations of thyroid status include prenatal and maternal radiothyroidectomy during pregnancy, postnatal thyroidectomy, the use of antithyroid drugs, and the administration of thyroid hormones (thyroxine, triiodothyronine, and thyrotropic releasing hormone).